The difference between the two yakuman forms are subtle. Shousuushi only need a complete group of three out of the four wind types, while pairing up the fourth type. On the other hand, daisuushi requires a complete group for all four wind types.
Shousuushi and daisuushi both have the value of a yakuman and therefore award 32000 points to a non-dealer winner or 48000 points to a dealer winner. However, as daisuushi is a rarer yakuman, in rulesets where a double yakuman is allowed, it is counted as such and awards 64000 points to a non-dealer winner or 96000 points to a dealer winner respectively.